Among the fuel cells of greatest interest in applications relating to the motor vehicle or car sector are solid polymer electrolyte fuel cells.
In a solid electrolyte fuel cell, the polymer solid electrolyte is a proton exchange membrane. Such membranes must have a low permeability to reactant gases (e.g. H2, CH4 and O2) and a maximum electrical and catalytic efficiency. They must also have adequate conduction properties and a minimum ohmic drop under a high current density.
Materials which can serve as a basis for such membranes must mainly have the following chemical and electrochemical properties: stability of the plastic material in a reducing medium, oxidation stability and hydrolysis stability. The membrane must also have a good hydrothermal stability. The use of perfluorine acid ionomers such as e.g. NAFION® has been proposed as a proton exchange membrane for such applications.
For many membranes the conductivity of the membrane is very sensitive to the degree of hydration. When subject to rising temperatures and temperatures close to the boiling point of water, due to the decreasing dehydration of the membrane the problem arises of a reduction in its electrical conductivity and at the same time an increase in the fuel transfer permeability. This leads to a reduction in the performance characteristics or a deterioration of the membrane.
However, numerous advantages are associated with the increase in the operating temperature of a proton exchange membrane fuel cell, namely in the case of stationary applications the cogeneration of heat can be useful. For use as the motive energy source of a vehicle, such as road vehicles and more specifically cars, the use of fuel cells operating at a higher temperature makes it possible to reduce the heat dissipation capacity of the cooling system and therefore reduce the bulk thereof. A reduction in the bulk facilitates the integration thereof in the vehicle and decreases the price.